The present invention generally relates to controlling operation of a battery to assure termination of discharge of the battery before damaging the battery. It is known that discharge of a battery must be effected at or above a particular state of charge to avoid damaging the battery. In the past the state of charge for determining whether to terminate discharge of a battery has been determined based on a fixed voltage. However, battery terminal voltage when a load is applied to the battery is comprised of two elements: (1) a fixed battery voltage value based upon the open circuit voltage characteristic of the battery, and (2) internal impedance of the battery determined by current-resistance (IR) drop within the battery. A fixed battery termination voltage does not reflect changes in terminal voltage caused by a load being coupled with the battery. Further, a fixed battery termination voltage does not reflect changes in internal resistance of the battery that occur as the battery ages.
For purposed of this description, the term “age” is intended to refer to the number of charge/discharge cycles to which a battery has been subjected rather than the chronological amount of time that the battery has existed.
For a new battery the difference in state of charge is small with a charging current present as compared with an open circuit state of charge. However, the difference in states of charge can increase significantly as the battery ages because the internal resistance of the battery increases with age. By way of example and not by way of limitation, internal resistance of a lithium ion battery can increase by five times after 300 charge/discharge cycles.
A detrimental result of failing to account for change in internal IR drop within a battery as the battery ages and relying solely on a fixed termination voltage for the battery is that battery operation may be under-utilized. That is, terminating discharge of the battery at a fixed termination voltage may leave a significant amount of battery capacity unused. Because of the increase in internal battery impedance, up to about 30% of chemical capacity of a battery may be still available for use when discharge of the battery terminated solely based upon a fixed termination voltage.
There is a need for a method and apparatus for controlling discharge of a battery to avoid damage to the battery while maximizing use of battery capacity.